Reciprocal mode saw correlator method and apparatus

ABSTRACT

A surface-acoustic-wave correlator for decoding a spread-spectrum signal having a data signal modulated with a plurality of chip sequences and reciprocal-chip sequences. A tapped-delay-line has a plurality of taps defining a tapped-delay-line structure matched to the chip sequence. In response to a plurality of first chips and second chips embedded in the spread-spectrum signal, the tapped-delay line generates TDL-chip sequences and inverse-TDL-chip sequences. A first transducer is acoustically coupled to the tapped-delay-line. In response to the spread-spectrum signal modulated by the chip sequence, the first transducer correlates a first group of the plurality of TDL-chip sequences and inverse-TDL-chip sequences and outputs a first correlation pulse. A second transducer is acoustically coupled to the tapped-delay-line. In response to the spread-spectrum signal modulated by the reciprocal-chip sequence, the second transducer correlates a second group of the plurality of TDL-chip sequences and inverse-TDL-chip sequences and outputs a second correlation pulse. In response to the first correlation pulse and the second correlation pulse, a decision circuit outputs the first bit and the second bit, respectively.

This is a continuation of application Ser. No. 08/004,071 filed on Jan.13, 1993, now U.S. Pat. No. 5,355,389.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to spread-spectrum communications, and moreparticularly to a spread-spectrum signal processing technique throughthe use of a single phase shift encoded tapped-delay linesurface-acoustic-wave correlator (SAWC) to demodulate multiple phaseshift keyed (PSK) codes.

2. Description of Related Art

The effects of surface-acoustic waves applied to a piezoelectricmaterial to convert electrical energy to acoustic energy and vice versafor analog signal processing purposes have been known and practiced inthe prior art for many years. This process, as applied to phase codedSAW correlators, consists, in its most basic form, of an inputtransducer and a phase coded tapped-delay line matched to a phase shiftencoded carrier. In general, this operation is carried out by applyingan electrical signal to a transducer which consists of a sequence ofmetallised interdigital finger pairs deposited on the surface of apiezoelectric material. The transducer converts this electrical signalto an acoustic wave which propagates down the surface of the substrateto the tapped-delay line. Acoustic energy is converted to electricalenergy at the metallised delay-line taps. When the phase encoded wavematches the phase configuration of the delay line taps, the electricalsignals are added in phase with each other, and a correlation signal,which provides a signal to noise improvement, is generated and coupledto other electronic circuits through the busses of the tapped-delayline. Multiple correlations may be accomplished by placing separatelyencoded tapped-delay lines in parallel on the same substrate. Distinctcorrelation pulses will then occur upon application of matched phaseshift encoded signals at the transducer input.

The reciprocal properties of SAW devices allow for this process to takeplace in reverse, where the tapped-delay line is excited by a phasecoded electrical signal, and a correlation signal will occur at theoutput of the transducer. These reciprocal properties have beendiscussed for many years in literature and conferences. Certain kinds ofdevices and signal formats have shown more promise in this area thanothers. In 1973 J. Burnsweig of Hughes Corp. published a paper detailingthe use of linear FM pulse compression matched filters operating inreciprocal manner ("Ranging and Data Transmission Using Digital EncodedFM Chirp Surface Acoustic Wave Filters", IEEE Transactions in MicrowaveTheory, Vol. MTT-21, pp 272-279, April 1973). This approach involvesexciting the long tapped-delay line with the linear FM encoded signaland utilizing the transducers, located a certain distance away from eachend of the delay line, as the elements that coherently sum the waveformsegments to produce a compressed pulse. The reciprocal approach with thelinear FM chirp waveform was utilized to differentiate between a "one"bit and a "zero" bit for satellite ranging/data transmissionapplications.

While one and zero bit differentiation has been applied toward a numberof phase shift keyed (PSK) waveforms, most of these approaches appear toinvolve some form of acoustoelectric convolver and a hybrid network. Thesimpler approach presented in this invention employs a SAW BPSK matchedfilter configuration with two transducers located near the ends of aphase coded tapped-delay line. As briefly described above, the twotransducers are typically utilized as inputs either to assist ingenerating the BPSK sequence or serve as the input for the BPSK encodedwaveform, and the tapped-delay line serves as the summing network togenerate the correlation peak. The reciprocal approach involves the useof the SAW tapped-delay line as the input structure for a one bit codeand a reciprocal code representing the zero bit. Coherent summation ofthe BPSK sequence can be sensed, at a minimum within a chip width fromone edge of the tapped-delay line. The summation can be sensed by anappropriate transducer structure that has dimensions corresponding toone chipwidth along with having the correct interdigital finger spacingfor the center frequency.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method for demodulatingmultiple data bits from a phase code sequence with a singletapped-delay-line surface-acoustic-wave correlator.

Another object of the invention is to provide a spread-spectrum receiverrequiring no reference code synchronization.

A further object of the invention is to provide spread-spectrumdemodulation through the use of single tapped-delay-line SAW correlatorswhich may be manufactured with highly distinctive codes.

Another object of the invention is to provide a spread-spectrumdemodulator which provides a lower bandwidth to processing gain ratioand more code variability than a linear FM chirp system. The BPSKencoded sequence is more robust with respect to bandwidth narrowing andsome manufacturing tolerance variations compared with .the linear FMchirp waveform.

According to the present invention, as embodied and broadly describedherein, a system using a surface-acoustic-wave correlator for decoding aspread-spectrum signal having a data signal modulated with a pluralityof chip sequences and reciprocal-chip sequences is provided comprisingcommunications channel, data-sequence-generating means,chip-sequence-generating means, chip-sequence-controlling means, signalmeans, carrier-modulating means, power means, front-end means,tapped-delay-line means and decision means. The data-sequence-generatingmeans, chip-sequence-generating means, chip-sequence-controlling means,signal means, carrier-modulating means, power means, front-end means,tapped-delay-line means and decision means may be embodied as a datadevice, a code generator, a chip-sequence controller, a signal source, aproduct device, a power device, a receiver-front end, a tapped-delayline and a decision/detector circuit, respectively.

The data device generates a data-bit sequence having first bits andsecond bits. The code generator repetitively generates a chip sequencehaving a plurality of first chips and second chips. The chip-sequencecontroller outputs the chip sequence in response to each first bit, andoutputs the reciprocal-chip sequence in response to each second bit. Byreciprocal-chip sequence is meant a time reversed version of the chipsequence. The signal source generates a carrier signal. The productdevice generates the spread-spectrum signal by phase modulating thecarrier signal with the chip sequence and reciprocal-chip sequence. Thepower device sends the spread-spectrum signal over the communicationschannel, and optionally helps to limit a power level of thespread-spectrum signal to less than a predetermined-threshold level atthe tapped-delay line.

The receiver-front end receives the spread-spectrum signal. Thetapped-delay line has a first end and a second end. The tapped-delayline also has a plurality of taps defining a tapped-delay-line structurephase-matched to the chip sequence. The tapped-delay line generates aplurality of TDL-chip sequences and inverse-TDL-chip sequences, inresponse to each of the plurality of first TDL chips and second TDLchips embedded in the spread-spectrum signal, respectively. A TDL chipis defined as a segment of the carrier signal of length equivalent to aperiod of each chip generated by the chip generator, with a first TDLchip having a first phase, and a second phase of a second TDL chipshifted with reference to the first phase.

A first transducer is coupled acoustically to the first end of thetapped-delay line. The first transducer correlates a first sequence ofthe plurality of TDL chips and inverse-TDL chips generated by thetapped-delay line and outputs a first correlation pulse, in response tothe spread-spectrum signal modulated by the chip sequence. A secondtransducer is coupled acoustically to the second end of the tapped-delayline. The second transducer correlates a second sequence of theplurality of TDL chips and inverse-TDL chips generated by thetapped-delay line and outputs a second correlation pulse, in response tothe spread-spectrum signal modulated by the reciprocal-chip sequence.

A decision/detector circuit outputs the first bit and the second bit inresponse to detecting the first correlation pulse and the secondcorrelation pulse, respectively.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention also may be realized andattained by means of the instrumentalities and combinations particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate preferred embodiments of theinvention, and together with the description serve to explain theprinciples of the invention.

FIG. 1 illustrates a system using the reciprocal SAWC in aspread-spectrum receiver according to the present invention;

FIG. 2A is an equivalent block diagram for a SAWC BPSK matched filtershowing a chip sequence generated by a 1-bit;

FIG. 2B shows the correlation values at the 1-bit transducer;

FIG. 2C shows the correlation values at the 0-bit transducer;

FIG. 3A is an equivalent block diagram for a SAWC BPSK matched filtershowing a chip sequence generated by a 0-bit;

FIG. 3B shows the correlation values at the 0-bit transducer; and

FIG. 3C shows the correlation values at the 1-bit transducer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention disclosed in this patent is related to the inventionsdisclosed in U.S. patent application entitled "Spread SpectrumCorrelator", by Robert C. Dixon and Jeffrey S. Vanderpool and havingSer. No. 07/390,315 and Filing Date of Aug. 7, 1989, now U.S. Pat. No.5,022,047 in U.S. patent application entitled "Asymmetric SpreadSpectrum Correlator" by Robert C. Dixon and Jeffrey S. Vanderpool andhaving Ser. No. 07/389,914 and Filing Date of Aug. 7, 1989, now U.S.Pat. No. 5,016,255 and in U.S. patent application entitled "SAWC PhaseDetection Method and Apparatus" by Robert C. Dixon and having Ser. No.07/556,147 and Filing Date of Jul. 23, 1990, now abandoned which areincorporated herein by reference.

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals indicate likeelements throughout the several views.

The present invention includes a system using a surface-acoustic-wavecorrelator for decoding a spread-spectrum signal having a data signalmodulated with a plurality of chip sequences and reciprocal-chipsequences. The system comprises a communications channel,data-sequence-generating means, chip-sequence-generating means,chip-sequence-controlling means, signal means, carrier-modulating means,power means, front-end means, tapped-delay-line means and decisionmeans.

As illustratively shown in FIG. 1, the data-sequence-generating means,chip-sequence-generating means, chip-sequence-controlling means, signalmeans, carrier-modulating means, power means, front-end means,tapped-delay-line means and decision means, by way of example, may beembodied as a data a device 13, a code generator 14, a chip-sequencecontroller 11, an RF signal source 16, a phase modulator 12, a powerdevice 21, a receiver-front end 17, a surface-acoustic-wave correlator18, and a decision/detector circuit 19, respectively.

The chip-sequence controller 11 is coupled to the data device 13 and thecode generator 14. The phase modulator 12 is coupled to the RF signalsource 16 and the chip-sequence controller 11. The power device 21 iscoupled to the product device 12.

The receiver-front end 17 is coupled to the communications channel 15.The surface-acoustic-wave correlator 18 is coupled to the receiver-frontend 17. The decision/detector circuit is coupled to thesurface-acoustic-wave correlator 18.

The data device 13 outputs a data-symbol sequence. The data-symbolsequence usually includes information to be communicated by thespread-spectrum signal. The data-symbol sequence may have each datasymbol represent two or more data bits. In a binary case, thedata-symbol sequence has each data symbol represent one data bit, andaccordingly, the data-symbol sequence is known as a data-bit sequence.The data-symbol sequence, for example, may be a data-bit sequence havingfirst bits and second bits, which are the 1-bit and 0-bit. As anexample, the data device 13 may be a computer terminal, a device whichhas converted analog inputs such as voice, audio or video, to data, orany other source where data are to be transmitted from a transmitter toa receiver.

The code generator 14 repetitively generates a chip sequence having aplurality of first chips and second chips. The first chips and secondchips are commonly known as 1s and 0s. The repetitively generated chipsequence is known as the spreading sequence for generating thespread-spectrum signal. In a preferred embodiment, the chip sequence isa pseudo-noise (PN) code. The code generator 14 may employ shiftregisters having appropriate taps for generating the chip sequence.

For the binary case, the chip-sequence controller 11 outputs the chipsequence from the code generator 14 in response to each first bitreceived from the data device 13, and outputs the reciprocal-chipsequence in response to each second bit received from the data device13. Accordingly, the chip-sequence controller 11 outputs a concatenatedplurality of chip sequences and reciprocal-chip sequences, in responseto a concatenated plurality of first bits and second bits from datadevice 13.

For the binary case, the chip-sequence controller 11 causes a shiftregister containing the chip sequence to shift in a forward directionfor each first bit, and in the reciprocal (opposite) direction for eachsecond bit. Thus, chip-sequence controller 11 outputs a chip sequence inthe case of a data 1-bit, and a reciprocal-chip sequence for data 0-bit.

By reciprocal-chip sequence is meant a time reversed version of the chipsequence. By way of example, if the chip sequence is 110101, then thereciprocal-chip sequence is 101011. Preferably, a complete sequence ofthe repetitively generated chip sequence or reciprocal-chip sequence isoutputted from the chip-sequence controller 11 for each data symbol. Achip sequence optionally may be generated coherently with each datasymbol of the data-symbol sequence, and each data symbol determineswhether a chip sequence or its reciprocal is generated.

The signal source 16 generates a carrier signal. The term "carriersignal" is defined herein to be any signal at an RF, intermediatefrequency (IF) , or other frequency at which the surface-acoustic-wavecorrelator 18 operates. The center frequency of the carrier signal ismatched to the surface-acoustic-wave correlator 18 used at the receiver.

The carrier-modulating means is coupled to the chip-sequence-controllingmeans and the signal source 16, and may be embodied as a product deviceor, as illustrated in FIG. 1, a phase modulator 12. The phase modulator12 generates the spread-spectrum signal by phase modulating the carriersignal from the signal source 16 with the plurality of chip sequencesand reciprocal-chip sequences from the chip-sequence controller 11,causing phase shifts in the carrier signal corresponding to each statetransition of the chip sequence. The spread-spectrum signal is thecarrier signal modulated with the output from the chip-sequencecontroller 11. The phase modulator 12 outputs the spread-spectrum signalto the power device 21.

The power device 21 sends the spread-spectrum signal over thecommunications channel 15, and limits a power level of thespread-spectrum signal to less than a predetermined-threshold level atthe input to the surface-acoustic-wave correlator 18. The power device21 is optional, and includes any power amplifier and/or power limiter.Typically, the power device 21 is coupled to a communications channelfrom the surface-acoustic-wave correlator 18. The transmitter power isadjusted to help maintain the power level at the input to thesurface-acoustic-wave correlator 18 to below a predetermined-thresholdlevel which prevents the surface-acoustic-wave correlator 18 fromoperating in a non-linear range. In some commercially available devices,the predetermined-threshold level has been found to be less than 20 dBm.

The communications channel 15 may be any medium where thespread-spectrum signal may propagate or travel.

The receiver-front end 17 receives the spread-spectrum signal from thecommunications channel 15. The receiver-front 17 end includes anyantenna, amplifier and/or impedance matching circuitry coupling thesurface-acoustic-wave correlator 18 to the communications channel 15.

The present invention includes a phase coded surface-acoustic-wavecorrelator 18 for demodulating a received spread-spectrum signal. Thesurface-acoustic-wave correlator 18 comprises tapped-delay-line means,first transducer means and second transducer means. As illustrativelyshown in FIGS. 2A and 3A, the tapped-delay-line means, first transducermeans and second transducer means may be embodied as tapped-delay line30, first transducer 36, and second transducer 37. The spread-spectrumsignal has a data signal embedded in a carrier signal by phasemodulating the carrier signal with a chip sequence and a reciprocal-chipsequence, as previously described. The received spread-spectrum signalis applied to the tapped-delay-line bus, which serves as thesurface-acoustic-wave correlator 18 input. The tapped-delay line busconverts the electrical signal received to an acoustic signal. When aphase-matched-chip sequence is received at the surface-acoustic-wavecorrelator 18, an output transducer will output a correlation pulse,which is applied to the detection means, which may be embodied as anamplitude and/or phase detector 19.

In a tapped-delay line, as the electrical signal is converted toacoustical energy, an acoustic wave propagates on the surface of asubstrate, with each chip width section of the wave adding in or out ofphase with the delay line fingers. When the acoustic waves reach theoutput transducers at each end of the tapped-delay line, the phasecomponents of the wave are summed to create a correlation pulse whoseamplitude is in direct proportion to the number of phase matches of thedelay line. The output transducers convert this acoustic energy toelectrical energy and output the correlation pulse in the form of anamplitude modulated RF signal of frequency equivalent to the centerfrequency of the surface-acoustic-wave correlator 18 and the receivedspread-spectrum signal.

More particularly, as shown in FIGS. 2A and 3A, the tapped-delay-line 30has a plurality of taps defining a tapped-delay-line structure. In theexemplary arrangement shown, the tapped-delay line 30 has five taps 31,32, 33, 34, 35. The tapped-delay-line structure has the taps adjusted toprovide a phase match with a received spread-spectrum signal modulatedby the chip sequence or reciprocal-chip sequence.

The tapped-delay line 30 has a first end and a second end. The firsttransducer 36 is acoustically coupled to the first end of thetapped-delay line 30. The second transducer 37 is acoustically coupledto the second end of the tapped-delay line 30. The tapped-delay line 30generates a plurality of TDL-chip sequences and inverse-TDL-chipsequences, in response to each chip of the plurality of first chips andsecond chips embedded in the spread-spectrum signal, respectively.

FIGS. 2A, 2B, and 2C illustrate the case for a received spread-spectrumsignal being a 1-bit, which is represented by the chip sequence 11101.The received spread-spectrum signal is applied to the tapped-delay-linebus, which serves as the input to the tapped-delay line 30. Each 1-chipof the chip sequence 11101 generates in the tapped-delay line atapped-delay-line-chip sequence (TDL-chip sequence), 11101, and each0-chip generates in the tapped-delay line an inverse-TDL-chip sequence,00010. The generation of each TDL-chip sequence and inverse-TDL-chipsequence is delayed in time by the time equivalent of one chip. FIG. 2Bshows a group of TDL-chip sequences and an inverse-TDL-chip sequences asthey propagate as acoustic waves toward the first end of thetapped-delay line 30. The group shown in FIG. 2B is in response to thechip sequence 11101, which represents the 1-bit. FIG. 2C shows theoutput 61 of the first transducer 36, illustrated as a 1-bit transducer,which is the sum of the chips at any point in time. The 1-chips addin-phase with the TDL structure and are given a value of +1 and the0-chips add out-of-phase with the TDL structure and are given a value of-1. The output 61 shows that the first transducer 36 generates a maximumvalue of 5, and accordingly the first correlation pulse is generatedwhen the total of the output 61 reaches 5.

FIG. 2B shows a group of TDL-chip sequences and an inverse-TDL-chipsequence as they propagate as acoustic waves toward the second end ofthe tapped-delay line 30. The group shown in FIG. 2B is in response tothe chip sequence 11101, which represents the 1-bit. The output 51 ofthe second transducer 37, illustrated as a 0-bit transducer, is the sumof the chips at any point in time. The 1-chips add in-phase with the TDLstructure and are given a value of +1 and the 0-chips add out-of-phasewith the TDL structure and are given a value of -1. The output 51 showsthat the second transducer 37 does not generate the maximum level, sincethe levels are below a maximum value.

The decision/detector circuitry 19 detects which output of the firsttransducer 36 and second transducer 37 produced the maximum value, andthereby outputs a 1-bit if the maximum value is from the firsttransducer 36.

FIGS. 3A, 3B, and 3C illustrate the case for a received spread-spectrumsignal being a 0-bit, which is represented by the reciprocal-chipsequence 10111. The received spread-spectrum signal is applied to thetapped-delay-line bus, which serves as the input to the tapped-delayline 30. Each 1-chip of the chip sequence 10111 generates atapped-delay-line-chip sequence (TDL-chip sequence), 11101, and each0-chip generates an inverse-TDL-chip sequence, 00010. The generation ofeach TDL-chip sequence and inverse-TDL-chip sequence is delayed in timeby the time equivalent of one chip. FIG. 3B shows a group of TDL-chipsequences and an inverse-TDL-chip sequence as they propagate as acousticwaves toward the second end of the tapped-delay line 30. The group shownin FIG. 3B is in response to the reciprocal-chip sequence 10111, whichrepresents the 0-bit. The output 51 of the second transducer 37 is thesum of the chips at any point in time. The 1-chips add as a +1 and the0-chips add as a -1, as stated previously. The output 51 shows that thesecond transducer 37 generates a maximum level of 5, and accordingly thesecond correlation pulse is generated when the total of the output 51reaches 5.

FIG. 3C shows a group of TDL-chip sequences and an inverse-TDL-chipsequence as they propagate as acoustic waves toward the first end of thetapped-delay line 30. The group shown in FIG. 3C is in response to thereciprocal-chip sequence 10111, which represents the 0-bit. The output61 of the first transducer 36 is the sum of the chips at any point intime. The 1-chips add as a +1 and the 0-chips add as a -1, as statedpreviously. The output 61 shows that the first transducer 36 does notgenerate the maximum value, since the values are below the maximumvalue.

The decision/detector circuitry 19 detects which output of the firsttransducer 36 and second transducer 37 produces the maximum value, andthereby outputs a 0-bit if the maximum value is from the secondtransducer 37.

Accordingly, the first transducer 36 correlates a first group of theplurality of TDL-chip sequences and inverse-TDL-chip sequences generatedby the tapped-delay line 30 and outputs a first correlation pulse, inresponse to the spread-spectrum signal modulated by the chip sequence.The first transducer 36 at the first end of the tapped-delay line 30produces a first correlation pulse representing a data 1-bit, inresponse to the received spread-spectrum signal modulated with the chipsequence. Similarly, the second transducer 37 correlates a second groupof the plurality of TDL-chip sequences and inverse-TDL-chip sequencesgenerated by the tapped-delay line 30 and outputs a second correlationpulse, in response to the spread-spectrum signal modulated by thereciprocal-chip sequence. The second transducer 37 at the second end ofthe tapped-delay line produces a second correlation pulse representing adata 0-bit in response to the received spread-spectrum signal modulatedwith the reciprocal-chip sequence.

A decision/detector circuit 19 outputs the first bit and the second bitin response to detecting the first correlation pulse and the secondcorrelation pulse at the outputs of the first transducer 36 and secondtransducer 37, respectively.

The present invention further includes a method using asurface-acoustic-wave correlator having a tapped-delay line for decodinga spread-spectrum signal having a data signal modulated with a pluralityof chip sequences and a reciprocal-chip sequences. The method comprisesthe steps of: generating a plurality of TDL-chip sequences andinverse-TDL-chip sequences with the tapped-delay line in response to aplurality of first chips and second chips embedded in thespread-spectrum signal matching the taps of the tapped-delay line;correlating a first group of the plurality of TDL-chip sequences andinverse-TDL-chip sequences generated by the tapped-delay line;outputting a first correlation pulse from a first transducer in responseto the spread-spectrum signal being modulated by the chip sequence;correlating a second group of the plurality of TDL-chip sequences andinverse-TDL-chip sequences generated by the tapped-delay-line;outputting a second correlation pulse from the second transducer inresponse to the spread-spectrum signal being modulated by thereciprocal-chip sequence; and outputting from a decision circuit thefirst bit and the second bit in response to the first correlation pulseand the second correlation pulse being outputted from the firsttransducer and the second transducer, respectively.

It will be apparent to those skilled in the art that variousmodifications can be made to the system using a surface-acoustic-wavecorrelator or other analog correlators, a including but not limited tocharged-coupled devices, for decoding a spread-spectrum signal of theinstant invention with out departing from the scope or spirit of theinvention, and it is intended that the present invention covermodifications and variations of the system using thesurface-acoustic-wave correlator provided they come in the scope of theappended claims and their equivalence. Such modifications and variationsinclude, but are not limited to, applying the surface-acoustic-wavecorrelator to communications systems employing other type of phasemodulation such as QPSK and M-ary PSK.

In a preferred alternative embodiment, other forms of phase modulationmay be used, such as QPSK, OQPSK, MSK, SFSK, CPSM or "chirp" modulation.In QPSK, the system of FIG. 1 is operated in a manner similar to theBPSK case. The data device 13 outputs a data-symbol sequence, which maybe a data-bit sequence comprising 1-bits and 0-bits. The code generator14 may generate a spreading code or codes such as a PN code or codes forQPSK modulation of the data-symbol sequence. The chip sequencecontroller 11 may output a sequence of chip sequences andreciprocal-chip sequences in response to the data-symbol sequence.

The signal source 16 may generate a carrier signal, and the phasemodulator 12 may modulate the carrier signal with the sequence of chipsequences and reciprocal-chip sequences from the chip sequencecontroller 11, using QPSK. For example, it may select, in response totwo data bits, one of four phase delays for a segment of the carriersignal. (QPSK is well known in the art, and is described in detail inRobert C. Dixon, Spread Spectrum Systems.) The spread-spectrum signalmay comprise the carrier signal modulated by the output from the chipsequence controller 11 by means of QPSK, and may be transmitted to thepower device 21 and ultimately to the communications channel 15, inmanner similar to the BPSK case.

In QPSK, the receiver of FIGS. 2 and 3 is operated in a manner similarto the BPSK case. The receiver front end 17 may receive thespread-spectrum signal from the communications channel 15 and transmitthat signal to a tapped-delay-line 30 and ultimately to the firsttransducer 36 and the second transducer 37, in manner similar to theBPSK case. The received spread-spectrum signal may be applied to thetapped-delay-line bus, which may convert the electrical signal toacoustical energy. However, rather than simply adding acoustical energywhich is in-phase or out-of-phase, the QPSK signal may sum to acorrelation pulse which is in one of a plurality of phase relations. Thereceiver may determine, in response to the phase relation of the QPSKcorrelation pulse and in response to whether a chip sequence or areciprocal-chip sequence was received, a plurality of data bits.

In MSK, the system of FIG. 1 is again operated in a manner similar tothe BPSK case. The data device 13 outputs a data-symbol sequence, whichmay be a data-bit sequence comprising 1-bits and 0-bits. The codegenerator 14 may generate a spreading code such as a PN code for MSKmodulation of the data-symbol sequence. The chip sequence controller 11may output a sequence of chip sequences and reciprocal-chip sequences inresponse to the data-symbol sequence.

The signal source 16 may generate a carrier signal, and the phasemodulator 12 may modulate the carrier signal with the sequence of chipsequences and reciprocal-chip sequences from the chip sequencecontroller 11, using MSK. For example, it may select, in response to adata bit, one of two frequencies for a segment of the carrier signal.(MSK is well known in the art, and is described in detail in Robert C.Dixon, Spread Spectrum Systems.) The spread-spectrum signal may comprisethe carrier signal modulated by the output from the chip sequencecontroller 11 by means of MSK, and may be transmitted to the powerdevice 21 and ultimately to the communications channel 15, in mannersimilar to the BPSK case.

In MSK, the receiver of FIGS. 2 and 3 is operated in a manner similar tothe BPSK case. The receiver front end 17 may receive the spread-spectrumsignal from the communications channel 15 and transmit that signal to atapped-delay-line 30 and ultimately to the first transducer 36 and thesecond transducer 37, in manner similar to the BPSK case. The receivedspread-spectrum signal may be applied to the tapped-delay-line bus,which may convert the electrical signal to acoustical energy. However,rather than simply adding acoustical energy which is in-phase orout-of-phase, the MSK signal may sum to a correlation pulse which isdominated by one of a plurality of frequency components, which may befiltered by the receiver after it is reconverted back to electricalenergy. The receiver may determine, in response to the frequencycomponents of the MSK correlation pulse and in response to whether achip sequence or a reciprocal-chip sequence was received, a plurality ofdata bits.

In "chirp" modulation, the system of FIG. 1 is again operated in amanner similar to the BPSK case. The data device 13 outputs adata-symbol sequence, which may be a data-bit sequence comprising 1-bitsand 0-bits.

The signal source 16 may generate a carrier signal, and the phasemodulator 12 may modulate the carrier signal with a sequence offrequency sweeps, either from a first frequency to a second frequency,or in reverse, from the second frequency to the first frequency, usingchirp modulation. For example, it may select, in response to a data bit,either an upward frequency sweep or a downward frequency sweep. (Chirpmodulation is well known in the art, and is described in detail inRobert C. Dixon, Spread Spectrum Systems.) The spread-spectrum signalmay comprise the carrier signal modulated by means of chirp modulation,and may be transmitted to the power device 21 and ultimately to thecommunications channel 15, in manner similar to the BPSK case.

In chirp modulation, the receiver of FIGS. 2 and 3 is operated in amanner similar to the BPSK case. The receiver front end 17 may receivethe spread-spectrum signal from the communications channel 15 andtransmit that signal to a tapped-delay-line 30 (or transversal filter)and ultimately to the first transducer 36 and the second transducer 37,in a manner similar to the BPSK case. The received spread-spectrumsignal may be applied to the tapped-delay-line bus, which may convertthe electrical signal to acoustical energy. Acoustical energy may besummed by a delay line (operated as a dispersive filter) which sums theenergy over a range of frequencies, creating a correlation pulse.Accordingly, the acoustical energy may be summed and reconverted back toelectrical energy, which may be filtered or otherwise processed by thereceiver. The receiver may determine a data bit in response to the chirpcorrelation pulse and in response to whether the pulse involved anupward frequency sweep or a downward frequency sweep.

Alternative Embodiments

While preferred embodiments are disclosed herein, many variations arepossible which remain within the concept and scope of the invention, andthese variations would become clear to one of ordinary skill in the artafter perusal of the specification, drawings and claims herein.

We claim:
 1. A method for communication comprising the stepsof:generating a spread-spectrum signal, said spread-spectrum signalcomprising a chip sequence for each 1-bit of a data signal to betransmitted, and comprising a reciprocal of said chip sequence for each0-bit of said data signal, said chip sequence and said reciprocal chipsequence each comprising a series of first chips and second chips,transmitting said spread-spectrum signal, receiving said transmittedspread-spectrum signal, and decoding said received spread-spectrumsignal.
 2. The method of claim 1 wherein said step of decodingcomprises:generating, in response to said received spread-spectrumsignal, a tapped-delay-line chip sequence for each first chip in saidreceived spread-spectrum signal, and an inverse of saidtapped-delay-line chip sequence for each second chip in said receivedspread-spectrum signal, correlating a first group of saidtapped-delay-line chip sequences and said inverse tapped-delay-line chipsequences, and outputting a first correlation pulse in response thereto,correlating a second group of said tapped-delay-line chip sequences andsaid inverse tapped-delay-line sequences, and outputting a secondcorrelation pulse in response thereto, outputting either a 1-bit or a0-bit in response to said first and second correlation pulses.
 3. Themethod of claim 2 wherein the step of outputting either a 1-bit or a0-bit comprises the step of detecting the larger of said first andsecond correlation pulses, and outputting a 1-bit when one of said firstand second correlation pulses is larger, and a 0-bit when the other ofsaid first and second correlation pulses is larger.
 4. A correlator fordecoding a spread-spectrum signal having a data signal modulated with aplurality of identical first chip sequences and a plurality of identicalsecond chip sequences, each of said second chip sequences beingreciprocals of said first chip sequences, comprising:a tapped-delay-linehaving a plurality of taps defining a tapped-delay-line structurematched to the first and second chip sequences, responsive to aplurality of first chips and second chips embedded in thespread-spectrum signal, and capable of generating a plurality of firsttapped-delay-line chip sequences and a plurality of second chipsequences, each of said second tapped-delay-line chip sequences being aninverse of said first chip sequences, a first transducer capable ofcorrelating to a first group of the plurality of first tapped-delay-linechip sequences and second tapped-delay-line chip sequences, and capableof outputting a first correlation pulse, said first correlation pulsebeing in one of a first plurality of phase relations, a secondtransducer capable of correlating to a second group of the plurality offirst tapped-delay-line chip sequences and second tapped-delay-line chipsequences, and capable of outputting a second correlation pulse, saidsecond correlation pulse being in one of a second plurality of phaserelations, a decision circuit responsive to said first and secondcorrelation pulses, whereby said correlator outputs a plurality of databits in response to the phase relations of said first and secondcorrelation pulses.
 5. The correlator of claim 4 wherein said spreadspectrum signal is a quadrature phase-shift keyed signal.
 6. Acorrelator for decoding a spread-spectrum signal having a data signalmodulated with a plurality of chip sequences and reciprocal-chipsequences, comprising:a tapped-delay-line having a plurality of tapsdefining a tapped-delay-line structure matched to the chip sequence,responsive to a plurality of first chips and second chips embedded inthe spread-spectrum signal, and capable of generating a plurality offirst tapped-delay-line chip sequences and a plurality of secondtapped-delay-line chip sequences, each of said second tapped-delay-linechip sequences being an inverse of said first tapped-delay-line chipsequences, a first transducer capable of correlating to a first group ofthe plurality of first tapped-delay-line chip sequences and secondtapped-delay-line chip sequences, and capable of outputting a firstcorrelation pulse, a second transducer capable of correlating to asecond group of the plurality of first tapped-delay-line chip sequencesand second tapped-delay-line chip sequences, and capable of outputting asecond correlation pulse, a first filter connected to said firsttransducer, having as an input said first correlation pulse, and havingas an output a first filtered correlation pulse, a second filterconnected to said second transducer, having as an input said secondcorrelation pulse, and having as an output a second filtered correlationpulse, a decision circuit responsive to said first and second filteredcorrelation pulses, whereby said correlator outputs a plurality of databits in response to the frequency components of said first and secondfiltered correlation pulses.
 7. The correlator of claim 6 wherein saidspread spectrum signal is a continuous phase modulated signal.
 8. Thecorrelator of claim 7 wherein said spread spectrum signal is a minimumshift keyed signal.
 9. A receiver for receiving a spread-spectrum signalhaving a data signal modulated with a sequence of upward frequencysweeps and downward frequency sweeps, wherein an upward frequency sweepcomprises a frequency sweep from a first frequency to a secondfrequency, and a downward frequency sweep comprises a frequency sweepfrom said second frequency to said first frequency, said receivercomprising:a tapped-delay-line having a plurality of taps defining atapped-delay-line structure matched to the frequency sweep sequence,responsive to said upward frequency sweeps and downward frequency sweepsembedded in the spread-spectrum signal, said tapped-delay-line capableof generating a plurality of first tapped-delay-line chip sequences anda plurality of second tapped-delay-line chip sequences, each of saidsecond tapped-delay-line chip sequences being an inverse of said firsttapped-delay-line chip sequences, a first transducer, responsive to thespread-spectrum signal modulated by the frequency sweep sequence, andconfigured as a dispersive filter, said first transducer capable ofcorrelating to a first group of the plurality of first tapped-delay-linechip sequences and second tapped-delay-line chip sequences, and capableof outputting a first correlation pulse, a second transducer, responsiveto the spread-spectrum signal modulated by the frequency sweep sequence,and configured as a dispersive filter, said second transducer capable ofcorrelating to a second group of the plurality of firsttapped-delay-line chip sequences and second tapped-delay-line chipsequences, and capable of outputting a second correlation pulse, adecision circuit responsive to said first and second correlation pulses,and outputting a first data bit or second data bit in response thereto.10. The correlator of claim 9 wherein said spread spectrum signal is achirp signal.
 11. The correlator of claim 9 wherein said decisioncircuit compares said first and second correlation pulses and outputssaid first data bit if said first correlation pulse is larger than saidsecond correlation pulse, and outputs said second data bit if saidsecond correlation pulse is larger than said first correlation pulse.